1. Field of Invention
The invention concerns a structure arrangement comprising a plurality of surface regions having a relief structure which has an optical-diffraction effect, in particular for visually identifiable, optical security elements for value-bearing documents, for example bank notes, credit cards, passes or cheque documents, or other items to be safeguarded.
2. Description of the Prior Art
When using a structure arrangement of that kind, visually perceptible items of information can be communicated to a viewer by diffraction and/or refraction of incident ambient light. In the simplest case a structure arrangement of that kind is afforded by a rectilinear wave or corrugation structure which is provided on the surface of a surface region of a carrier element and at which incident ambient light is reflected with diffraction and/or refraction. In this respect, the term wave or corrugation structure does not necessarily denote a structure with a surface line which is steady in terms of the cross-section of the surface region and which in particular is sinusoidal, but this may also involve rectangular, step-shaped or wedge-shaped surface structures.
Diffraction of incident light or light which passes through the structure arrangement, at the relief structures of the surface regions, and therewith the information which is emitted therefrom in the form of an optical diffraction image are determined by the number of wave or grating lines per unit of length of a surface region, the so-called spatial frequency, and by the orientation and the cross-sectional shape of the relief structure which is determined inter alia by the differences in respect of height in the relief structure, more specifically both by the differences in respect of height between the individual raised portions relative to each other, and also between raised portions and troughs or depressions of the relief structure. The relief structures of the surface regions can be of such a configuration and the surface regions can be so arranged that a given item of information can be emitted in a given viewing angle range and thus perceived by a viewer, whereas, in another viewing angle range, another item of information can be perceived.
An item of visually perceptible information which corresponds to the relief structure of the surface regions and which is dependent inter alia on the lighting or viewing angle, in particular information in regard to authenticity of the safeguarded item, can be communicated to a viewer in the form of the reflected light or the light which passes through the structure.
By virtue of the use of per se known security elements with a structure arrangement having an optical-diffraction effect, in regard to the articles to be safeguarded as were referred to in the opening part of this specification, it is possible for items of authenticity information in respect of the safeguarded article to be rendered visible even to the unpractised lay person, and at the same time for forgery, for example in the form of duplication, having regard to known forgery procedures, in particular optical duplication procedures, to be rendered impossible or made sufficiently difficult.
It is known for example to provide surface regions having a respective relief structure which is determined by the above-mentioned parameters--spatial frequency, orientation and cross-sectional shape of the relief structure, and differences in respect of height in the relief structure--, the relief structure being of dimensions which can still be perceived separately from each other by the naked eye. By virtue of a suitable configuration and orientation of the respective relief structure of the surface regions, it is possible for a given item of optical information which originates from one surface region to be communicated to a viewer in dependence on the lighting direction, in a given viewing angle range, while another item of visually perceptible information originates from another surface region, in the same viewing angle range. Pivotal movement of the carrier element which carries the structure arrangement, about an axis which is in the plane of the carrier element or about an axis which extends perpendicularly to the plane of the carrier element causes a change in the information which comes from the surface region which is first viewed--in particular that surface region can appear dark--while another surface region which initially appeared dark imparts optical information, for example in the form of a colour impression. Thus, by virtue of a suitable configuration of a relief structure which is periodic at least in a portion-wise manner, it is possible for virtually the entire radiation power which impinges on a surface region from a lighting direction to be diffracted away into the first and the minus-first diffraction order so that an item of optical information coming from that surface region can be perceived only within two closely restricted viewing angle ranges--the first and the minus-first diffraction orders--while the surface region appears dark in other viewing directions.
Admittedly, in the case of structure arrangements with surface regions which can be resolved separately by the naked eye, a viewer can receive items of information which vary in dependence on the lighting and viewing angles, but the surface regions which emit those items of information are perceived separately from each other. Therefore, macroscopically separate surface regions which change and light up in a grid-like form appear to a viewer. That is found to be disadvantageous for example when a relatively large surface portion of the structure arrangement, which comprises a plurality of surface regions, is intended to communicate a homogenous image impression and if therefore that surface portion is to appear in a first viewing angle range in a colour shade which is uniform over the extent of the surface portion, whereas a different image impression which is homogenous over the extent of the surface portion is intended to be perceptible in another viewing angle range.
Structure arrangements with surface regions each having a given relief structure, which can be resolved separately with the naked eye, can also have a disadvantageous effect by virtue of the fact that the magnitude of their diffraction orders, that is to say the viewing angle range which is associated with a diffraction order, is very small, and therefore a given item of information is visible only within a very small viewing angle range. That may be undesirable in individual cases.
Admittedly, it has been proposed in EP 0 330 738 B1 that the size of the surface regions is reduced, more specifically to a largest dimension of less than 0.3 mm. EP 0 375 833 B1 also discloses, in the structure arrangement, the provision of grid pattern areas which have a largest dimension of less than 0.3 mm and which include a plurality of area portions each having a mutually different grating structure. Admittedly, with structure arrangements of that kind, a relatively large surface portion can provide for the communication in a highly homogenous manner of various items of visually perceptible information, in dependence on the viewing angle; for that purpose however it is necessary to provide different relief structures within very small surface regions.